Great season premiere leads right into spoilers for ep 709 "Power Trip"
A young woman is found murdered, eclectrocuted through her ring on her finger by something. H, Cal, Eric and Tripp quickly realize the M.O.; it is that of the "Necklace Killer" 8 years ago a killer stalked and murdered young college students by electrocuting them through a point on their finger, usually jewelry, while using some point on their neck to ground them.
H was on the task force that investigated the murders but the killer was never caught. Now it seems as though he is back and the team seeks out the head investigator, Reggie Colson, from 8 yrs ago who is convinced he knows who the killer is but was never able to prove it. But as they investigate, the evidence isn't really leading them to suspect, Max Purdue, who looked good 8 yrs ago...it is leading them in another direction but Reggie Colson is sticking to the notion that it is Purdue and won't leave him alone.
Now it is a race against time to figure out if Purdue is really the killer or if it is someone else before the killer claims another victim.